Depending on gas field conditions natural gas streams of methane and other volatile hydrocarbons as first component are mixed with varying amounts of a second component such as CO2 (carbon dioxide).
Traditionally, CO2 (carbon dioxide) is removed from natural gas streams by selective dissolving the CO2 (solute) in a solvent. The solute and the solvent are contacted in a counter-current column (packed/trays). However, one of the disadvantages of such an absorption process is that it becomes less efficient when the CO2 concentration in the feed stream exceeds 30 mole %.
Moreover, the CO2 laden solvent is regenerated at low pressure (close to atmospheric pressure) such that a large volume of CO2 gas is produced. With more stringent CO2 emission regulations to come, this large CO2 volume exhibits a big problem for further handling (i.e., sequestration/further processing).
For larger CO2 contents in natural gas cryogenic (>30 mole %) cooling methods are available which (partially) liquefy the CO2 rich natural gas stream using refrigeration and subsequently feed said liquefied stream to a fractionation column to strip out the light end hydrocarbon fractions by re-boiling the bottom stream such that a liquefied CO2 enriched stream exits the bottom of the column In the column top the gas overhead is cooled such that a CO2 depleted gaseous stream is produced and a cold liquid is refluxed to the top tray of the column The main disadvantage of such cooling and fractionation process is that CO2 left in the gas overhead is still >15 mole % which requires further treatment (e.g., by absorption processes). Another disadvantage is the relatively large amount of heavier hydrocarbons which are lost via the column bottom stream. More sophisticated cryogenic processes comprise advanced columns which can operate in the solid phase of CO2 and in that way create a pure CO2 stream (since solidified CO2 does not contain hydrocarbons). Said advanced columns can produce a gas overhead with <2% mole CO2 due to the operation of these columns at extremely low temperatures (˜−85° C.). Therefore, the main disadvantage of these sophisticated cryogenic processes is the excessive cooling duties required to operate the column at these low temperatures.
More recently, membrane technology has been applied for selective removal of CO2 from natural gas streams. The CO2 is dissolved in the top layer of the membrane and transported through the membrane by diffusion hence driven by difference in partial CO2 pressure between the feed and permeate side of the membrane. The main disadvantage of membranes is the relatively high slip stream of light hydrocarbons which remains in the CO2 enriched, low pressure permeate stream. This permeate stream therefore requires further treatment by boosting the pressure and recycle said permeate to the feed side of the membrane or to a second stage membrane system. The latter causing a further increase in the required membrane surface. As a consequence the total required membrane surface to treat large amounts of CO2 in natural gas is enormous and therefore practically impossible for large gas fields. A further disadvantage of membranes is their sensitivity to heavier hydrocarbons, water and fine solid matter, which will significantly reduce the flux and selectivity (i.e., performance) of the membrane over time. To counteract said fouling problems large scale pre-treatment processes are required for robust membrane operations.